1. Field of the Invention
This invention relates to light emitting phosphor films with uniform and bright emission. In particular, the invention relates to a method of producing a cerium-doped SrS phosphor layers for thin film electroluminescent devices.
2. Description of Related Art
Thin film electroluminescent (TFEL) flat panel displays are used in applications where a wide viewing angle, wide temperature range and a rugged structure are important. All yellow emitting monochrome and red and green emitting multicolor TFEL displays existing on the market today are based on the ZnS:Mn light emitting phosphor.
Significant efforts have been and are being made to develop a blue emitting TFEL phosphor, which is necessary in order to realise a full colour TFEL display. The most promising phosphor for this purpose is SrS:Ce (R.O. Tornqvist, TFEL "Color by White", SID 1997 DIGEST, p. 855), which has a wide emission spectrum ranging from the blue spectral region to the red with a maximum near 500 nm.
The full color TFEL display structure embodies the ZnS:Mn and SrS:Ce phosphors stacked on top of each other. The primary colors are obtained by using color filters in the front. This "color by white" TFEL concept is adapted in both direct view and active matrix full color TFEL displays, which are under development.
SrS:Ce thin films have been fabricated by physical vapour deposition techniques such as Thermal Evaporation (under Molecular Beam Epitaxy or Hot Wall Deposition conditions), Electron Beam Evaporation, and Sputtering, by chemical vapour deposition techniques like Metalorganic Chemical Vapour Deposition and Atomic Layer Epitaxy (in the following abbreviated "ALE") and by mixed techniques such as Chemical Beam Epitaxy and Reactive Evaporation. Best results in terms of dot luminance and dot luminous efficiency have been achieved with Molecular Beam Epitaxy (MBE) (K. O. Velthaus, B. Huttl, U. Troppenz, R. Herrman and R. Mauch, New deposition process for very blue and bright SrS:Ce,Cl TFEL devices, SID 1997 DIGEST, p. 411). In these devices additional codoping is done with chlorine, manganese and silver with the aim of improving the stoichiometry and crystallinity of the SrS matrix and also to achieve charge compensation for the Ce.sup.3+ doping (by Ag.sup.+). Starting materials are Sr, S, CeCl.sub.3, Mn and Ag. The possibilities of upscaling this process for production are not known. Rather good results have been achieved with sputtered and Electron Beam Evaporation deposited SrS:Ce. In MOCVD Sr(thd).sub.2 and Ce(thd).sub.4 (wherein the abbreviation thd stands for 2,2,6,6-tetramethyl-3,5-heptanedionate) have been used for growing SrS:Ce EL films (R. Hiskes, S. A. DiCarolis, R. Muller-Mach, V. Mazzi, K. Nauka, and G. O. Muller, A Novel deposition Method for Thin Film Electroluminescent Devices, Extended Abstracts, 1.sup.st Int. Conference on the Science and Technology of Display Phosphors, San Diego, 1995, p. 137), but no significant results have been reported. Improved SrS:Ce TFEL performance has been claimed when bis(cyclopentadienyl) strontium, Sr(Cp).sub.2, and Ce(thd).sub.4 have been used instead of Sr(thd).sub.2 and Ce(thd).sub.4 because of higher vapor pressure and better controllability of Sr(Cp).sub.2 (U.S. Pat. No. 5,496,582, Mizutani K. et al.). Only qualitative results are given in the prior art.
Up till now the best SrS:Ce electroluminescent thin films by the ALE method have been made using Sr(thd).sub.2, Ce(thd).sub.4 and hydrogen sulfide as precursors (B. Soininen, M. Leppanen, and A. Pakkala, Bright and stable electroluminescent device based on SrS:Ce, 13th International Display Research Conference 1993, p. 233). This process has been used in fabricating multicolor thin film electroluminescent displays (M. Leppanen, G. Harkonen, A. Pakkala, E. Soininen and R. Tornqvist, Broadband double layer phosphor for an inverted filtered RGB electroluminescent display, 13th International Display Research Conference 1993, p. 229, Haaranen et al., 521(.times.3).times.256 RGB multicolor TFEL display based on "Color by white", SID 1995 Digest p. 883, and Harju et al., Bright 320(.times.3).times.240 RGB TFEL display based on "Color by white", SID 1997 Digest p. 859). In the growth of the phosphor film the surface is sequentially exposed to Sr(thd).sub.2 vapor and hydrogen sulphide, where first the Sr reactant is adsorbed on the surface and subsequently it reacts with H.sub.2 S for growing the host material, SrS. The dopant is added to the structure by periodically exposing the SrS surface sequentially to Ce(thd).sub.4 and hydrogen sulfide.
In EL display panel manufacturing good uniformity of the deposited thin films is essential both from a manufacturing cost and a panel performance point of view. The main issue in color TFEL panels made by ALE has been the strong luminance profile of the SrS:Ce film when e.g. Ce(thd).sub.4 is used for doping with cerium, especially because of a reduced luminance in the gas inlet part of the substrate. This is believed to be due to either the sticking and thermal decomposition properties of the precursor when reaching the substrate or to the incorporation of harmful additives transported to the surface via the precursor. The non-ideal SrS:Ce uniformity behaviour has been tracked down to be mainly due to the Ce precursor used.
A secondary issue is that typically cerium precursors used up till now at the temperature and pressure needed for suitable volatility have all been solid state, which is less desirable from a manufacturing process point of view. To reduce possible particle contamination, and also to minimise labour costs, liquid and gas sources would be preferred.
ALE is a powerful deposition method to tailor the dopant/host ratio in the films as molecular layer accuracy can be reached in placing the dopant into a desired position inside the host material in growth direction perpendicular to the substrate surface. This has been utilised in order to enhance the crystallinity of the host material by planar doping (Harkonen G. et al., Green emitting thin film electroluminescent device grown by Atomic Layer Epitaxy, SID 1990 Digest p. 232). It is more difficult to control the dopant concentration in the doping plane parallel to the substrate surface because, in such a case, the surface concentration is dependent on the precursor adsorption/desorption kinetics, sticking to the surface, reactivity and thermal stability. In the case of Ce, less than a monolayer adsorption is desirable. If there is interaction between adjacent Ce ions the color will shift towards higher wavelengths. Ce does not thermally diffuse in the SrS host at the used temperatures (&lt;520.degree. C.) and thus the excess of the precursor should be removed from the surface or the material pulse should be poor enough to begin with. Good uniformity cannot be achived using a thd precursor.
Volatile, thermally stable but on the other hand reactive enough precursors for Ce are not abundant. Several .beta.-diketonates and amide complexes tested have not produced enough luminance, not to mention an acceptable luminance uniformity in the ALE grown SrS:Ce. With other methods (R. H. Mauch, K. O. Velthaus, G. Bilger, and H. W. Schock, High efficiency SrS,SrCe:CeCl3 based thin film electroluminescent devices, J. Cryst. Growth 117 (1992) 964-968), it has been shown that by lowering the Ce content, the blue component can be enhanced. This has not been possible to realise with the Ce(thd).sub.4 precursor.